The present invention relates to the general field of making metal parts by a lost-wax casting method. The invention relates more particularly to preheating the shell molds that are used in such a casting method.
The lost-wax casting method consists of using wax to make an exact replica of the part that is to be fabricated. This model is covered by alternating and repeated dipping operations for building up a plurality of layers of ceramic in order to form a shell mold. After the wax has been eliminated, the shell mold is shaped with a cavity in which the smallest details of the part to be fabricated are reproduced. The “unwaxed” shell mold is then fired in a kiln, thereby giving it the mechanical properties it needs prior to having molten metal poured therein.
Furthermore, in order to avoid a thermal shock between the molten metal that is poured in at very high temperature (higher than 1000° C.) and the shell mold that receives it, the mold is subjected to a preheating operation that is likewise at high temperature (typically in the range 950° C. to 1200° C.). Once the shell mold has been preheated, it is placed in a pouring furnace in which it receives the molten metal. After the shell mold has cooled, it is destroyed and an exact copy of the wax model is thus obtained that is made out of metal.
It is known to perform the operation of preheating shell molds in gas furnaces that are dimensioned to receive a large number of shell molds. Generally, such furnaces are in the form of tunnel kilns into which the shell molds are charged for a typical duration of the order of six hours. More precisely, the shell molds are placed on bed plates mounted on carriages that are moved during a preheating cycle through the gas furnace from one of its ends to its other end. At the end of a preheating cycle, a plurality of shell molds are thus delivered from gas furnaces and can then be placed in the pouring furnace in order to receive the molten metal therein.
Such an operation of preheating shell molds by means of gas-fired tunnel kilns nevertheless presents numerous drawbacks. In particular, having recourse to organization of that type leaves no flexibility in managing production; a large quantity of shell molds are charged and it is not possible to change a temperature profile for a given batch (e.g. changing from a preheating temperature of 1100° C. to 950° C. on going from one shell mold to another).
It has also been found that the temperatures of shell molds within a single batch are not uniform on leaving gas furnaces, with temperature variations of plus or minus 15° C. relative to the setpoint temperature. Such non-uniformity may have the consequence of leading to metallurgical defects (of the crack type) in the parts that are to be fabricated.
Furthermore, having recourse to carriages that are movable in gas-fired tunnel kilns presents several disadvantages, such as considerable labor for installing and removing shell molds and a non-negligible risk of one or more shell molds breaking in such kilns.
Finally, gas-fired tunnel kilns have maintenance costs that are high, due in particular to the length of time needed to act on them, during which time the production means are completely unavailable.